A process of preparing unsaturated fatty acids, via unsaturated aldehydes, from olefin is a representative example of catalytic vapor-phase oxidation.
In partial oxidation of olefin, molybdenum oxide, and transition metal oxide are used to prepare a catalyst. As representative processes, there are a process of producing(meth)acrylic acid, via methacrolein, by oxidizing propylene or isobutylene, a process of producing phthalic anhydride by oxidizing naphthalene or ortho-xylene, and a process of preparing maleic anhydride by partially oxidizing benzene, butylene or butadiene
In the first step, propylene or isobutylene is oxidized by oxygen, diluted inert gas, water vapor, and a predetermined amount of catalyst, thereby mainly producing methacrolein. In step 2, the methacrolein is oxidized by oxygen, diluted inert gas, water vapor and a predetermined amount of a catalyst, thereby producing (meth)acrylic acid. A reactor used for such processes may be configured to perform both processes in one apparatus, or to perform each of the processes in a different apparatus.
(Meth)acrylic acid, which is reacted with alcohol, is mainly used to prepare (meth)acrylate used as a coating agent for paint, textile assistants, and paper. High-purity (meth)acrylic acid is used as a raw material for highly hygroscopic resins, demand for which has rapidly increased in recent years.
In general, a metal oxide catalyst is produced by coprecipitation reaction, hydrothermal synthesis, sol-gel synthesis, physical mixing reaction, etc. In such reaction processes, the metal oxide catalyst is precipitated in a polyanion, metal oxide, or metal hydroxylate form. Here, the physical properties and morphology of a precipitate are changed depending upon the pH, concentration, reaction temperature, and aging time of an aqueous solution, whereby the physical state, particle size, and crystal structure of the metal oxide catalyst are affected.
As examples of ligands bound to oxo anions and transition metal precursors which are used in catalysts for producing unsaturated fatty acid, there are —NH4, —NH2, —NOx, —Cl, —F, —N, —OH (hydroxyl), —SOx, —CO, —COO, —CnHmOx, alkoxide (O-Metal), and the like. Such ligands, which are essential ingredients for dissolving or purifying metal oxide, may be utilized as factors for changing physicochemical characteristics of a catalyst according to a suitable control method and thus controlling the activity of the catalyst.
In the related art, “Technology for Preparing Catalyst” introduced in Japanese Patent No. 4295521, a catalyst is prepared by powder-coating and firing a massive carrier. Here, the prepared catalyst is an acrylic catalyst characterized in that a mass reduction rate of a dried product thereof is 5 to 40% by mass at a catalyst drying temperature of 300° C. in an air atmosphere. However, such a preparation method causes structural change of the catalyst due to a relatively high drying temperature, thereby negatively affecting the performance of the catalyst and thus a conversion rate tends to be poor.
In addition, KR 10-0746971 B1 introduces a catalyst, which includes molybdenum and vanadium, and a catalyst poison in a content of 10 to 100 ppb measured by ion chromatography, further includes at least one volatile catalyst poison ingredient, and generates acrylic acid by catalytic vapor-phase oxidation between oxygen and acrolein, and a method of preparing acrylic acid, which includes a step of performing contact vapor-phase oxidation between oxygen and acrolein using the catalyst.
The catalyst, which is prepared by artificially adding a catalyst poison ingredient, i.e., aqueous ammonia, can lower hot spot temperature and inhibit reaction efficiency reduction accompanied by deterioration, thereby highly, stably maintaining an acrolein conversion rate for a long time. However, when a reducing substance, such as ammonia, is present in the catalyst, the reducing substance acts as a catalyst poison, thereby greatly increasing reaction temperature and, after a long period of operation, activating the catalyst. Accordingly, although the reducing substance can be used as a catalyst poison for controlling catalytic activity, there is considerable difficulty in quantitatively controlling the amount of the reducing substance in a process of producing the catalyst.
Meanwhile, treatment with an inorganic salt present in a catalyst precursor should be performed to be decreased during a process of preparing a catalyst. However, such an inorganic salt is additionally added, and thus, there is a disadvantage in that a process of removing the reducing substance is additionally required. Accordingly, there is a need for a technology for simply controlling, through sublimation, a type of ligands included in a catalyst and the amount thereof, when the catalyst is calcined, while providing superior reproducibility.